巨磁热材料相变过程晶格熵变和自旋熵变符号问题研究  

Sign of lattice and spin entropy change during phase transition in giant magnetocaloric materials

作　　者：郝嘉政 胡凤霞[1,2,3] 尉紫冰 沈斐然 周厚博 高怡红 乔凯明 梁文会 张丞 王晶 孙继荣[1,2,3] 沈保根 HAO Jia-Zheng;HU Feng-Xia;YU Zi-Bing;SHEN Fei-Ran;ZHOU Hou-Bo;GAO Yi-Hong;QIAO Kai-Ming;LIANG Wen-Hui;ZHANG Cheng;WANG Jing;SUN Ji-Rong;SHEN Bao-Gen(Beijing National Laboratory for Condensed Matter Physics,State Key Laboratory of Magnetism,Institute of Physics,Chinese Academy of Sciences,Beijing 100190,China;School of Physical Sciences,University of Chinese Academy of Sciences,Beijing 100049,China;Songshan Lake Materials Laboratory,Dongguan 523808,China;Fujian Innovation Academy,Chinese Academy of Sciences,Fuzhou 350108,China;Ganjiang Innovation Academy,Chinese Academy of Sciences,Ganzhou 341000,China)

机构地区：[1]中国科学院物理研究所磁学国家重点实验室,北京100190 [2]中国科学院大学物理科学学院,北京100049 [3]松山湖材料实验室,东莞523808 [4]中国科学院福建创新研究院,福州350108 [5]中国科学院赣江创新研究院,赣州341000

出　　处：《中国科学：物理学、力学、天文学》2021年第6期178-188,共11页Scientia Sinica Physica,Mechanica & Astronomica

基　　金：国家重点研发计划(编号:2019YFA0704900,2020YFA0711502,2018YFA0305704,2017YFA0206300,2017YFA0303601);国家自然科学基金(编号:U1832219,52088101,51771223,51971240,11921004,11934016);中国科学院战略性先导科技专项(B类)(编号:XDB33030200);中国科学院重点项目资助。

摘　　要：基于磁热效应的固态制冷技术由于绿色环保、高效节能等优势具有广阔的应用前景.具有巨大磁热效应的一级相变材料的共同特征是磁相变伴随晶胞参数或晶体对称性的不连续变化,其中一些材料体系在铁磁(FM)-顺磁(PM)相变过程中伴随晶格的负热膨胀.通常地,较大晶格体积相的声子振动模较软,进而具有较大的晶格熵.但实验和理论研究表明不同负热膨胀材料的声子振动模演化机制存在巨大差异.特别地,声子振动模在材料发生磁相变时的强化或软化决定相变时的晶格熵变的大小和符号.因此,具有负热膨胀特点的巨磁热材料相变过程中的晶格熵变和自旋熵变的符号相同还是相反一直存在争议.我们结合前人核共振非弹性X射线散射(NRIXS)等相关研究,并利用热流测量和德拜理论计算澄清了具有负热膨胀特点的巨磁热La(Fe, Si)_(13)基化合物和六角MM′X合金(M, M′=过渡族元素、X=主族元素)的晶格熵变和自旋熵变的符号问题.结果表明, La(Fe, Si)_(13)基化合物和MM′X合金在磁有序相变过程中晶格熵变和自旋熵变的符号始终相同,符合熵增原理.这项工作有助于全面理解具有负热膨胀行为的磁晶耦合材料巨磁热效应的内在物理机制及其多场调控原理.Solid-state refrigeration based on the magnetocaloric effect(MCE) has garnered worldwide attention because of its superior energy conservation and environment friendliness. A common feature of giant magnetocaloric materials is the simultaneous magnetic and lattice transitions, while some of them undergo negative expansion, i.e., lattice contraction,during the transition from the ferromagnetic(FM) to the paramagnetic(PM) phase. Generally, a larger lattice volume indicates softer phonons and therefore a larger phonon entropy. However, experimental and theoretical studies have shown that there are great differences in the mechanism of phonon modes for different materials. In particular, the strengthening or softening of the phonon vibration mode during magnetic phase transition determines the magnitude and sign of lattice entropy change. Therefore, for the giant MCE materials with negative thermal expansion, whether the sign of lattice and spin entropy change is the same or the opposite has always been controversial. Combined with previous studies of nuclear resonance inelastic X-ray scattering(NRIXS), and by means of heat flow measurements and Debye theory calculations, we have clarified the sign of lattice and spin entropy change of giant magnetocaloric La(Fe, Si)_(13)-based compounds and MM’ X alloys with negative thermal expansion. Results show that the lattice and spin entropy changes retain the same sign for La(Fe, Si)_(13)-based compounds and MM′X(M, M′= transition element, X=main element) alloys, which conforms to the principle of entropy increase. This work is helpful in fully understanding the intrinsic mechanism of giant magnetocaloric effect and its multi-field regulation for magnetostructural/magnetoelastic materials with negative thermal expansion during phase transition.

关 键 词：晶格熵变符号 自旋熵变符号 相变 负热膨胀 磁热效应 

分 类 号：O469[理学—凝聚态物理]